Hello,
300K is very, very ambitious. Unfortunately, I cannot tell you the price per square meter in your region, but here in the Stuttgart metropolitan area, you have to expect 350-600€ per sqm. A rule of thumb for house prices is approximately: upscale standard 500€ per cubic meter of enclosed space. Sure, if you "cut corners," you come to about 300€/sqm. But nothing is feasible below that. I speak from experience. My first plans were for 1500 sqm of enclosed space (I cannot estimate size). I obtained 15 offers, from prefab houses to solid construction (prefab house was by far the most expensive). The cheapest offer with "low cost" equipment without nonsense was 500K. Now the volume has been reduced to about 1100 sqm of enclosed space. Now it comes to about 400K.
Regards